The operation of a prior art scanning apparatus to which the present invention pertains will first be explained with reference to FIG. 8. A laser beam L emitted from a laser light source 1 is condensed into a predetermined beam shape by a cylindrical lens 2. The laser beam L is then incident onto the reflective surfaces of a regular-polygon, multi-faceted, mirror 3 that rotates in the direction indicated by the arrow R, and is then imaged by being passed through lens systems 4a and 4b so as to scan an object 5.
However, as shown in FIG. 9(a), in this type of optical scanning apparatus, air currents A, occur around the periphery of the mirror 3 as a result of its rotation in the direction R. As the air currents A, pass over an edge 3b formed by the intersection of two adjacent planar surfaces 3a, 3a the air separates from the downstream planar surface 3a. Where the air currents A1 are separated from the surface 3a, negative pressure is generated, which causes eddy currents A2 to develop within a region W on the planar mirror surface 3a. These eddy currents have a velocity component that is normal to the planar mirror surfaces, resulting in any dust particles that may be present in the air tending to become, over time, adhered to the mirror surface 3a in the area D as shown in FIG. 9(b) for one such mirror surface. The adhesion of dust particles in the area D, as can be seen in FIG. 9(b), occurs in a region that is centered on the mirror surfaces 3a in the axial direction of the rotating mirror. When a single laser beam is made to be incident onto the scanning apparatus, the central part of the mirror surface in the axial direction is an effective region in terms of reflecting the light in a scanning operation. Thus, when dust adheres to the region D, the reflectivity of this region drops dramatically, creating a problem in that the quality of the scanned output image declines.
In Japanese Laid-Open Patent Application H06-34908, technology is disclosed which prevents concentrated adhesion of dust on the mirror surfaces by providing a plurality of rectifying fins on the ends of the rotating multi-faceted mirror. In addition, in the disclosure of Japanese Laid-Open Patent Application H6-324279, technology is disclosed in which the rotating multi-faceted mirror and the motor that causes it to rotate together comprise a rotating unit, such that when the rotating multi-faceted mirror becomes soiled, the rotating unit itself can be replaced.
Among the conventional technologies described in the above-mentioned Japanese laid-open patent publications, the former technology is disadvantageous to making the mirror thin in the axial direction because the fins protrude in the axial direction. There also are concerns about it being difficult to attach the fins, which could lead to a loss in productivity. In addition, concern has also been raised that the fins cutting through the air could generate noise.
On the other hand, the latter technology causes the composition of the casing that houses the optical scanning apparatus to become complicated and costly, and it is also thought that such things as adjusting the optical axis of the rotating multi-faceted mirror after changing the rotating unit could be cumbersome.
The present invention relates to a rotating multi-faceted mirror of an optical scanning apparatus that scans at high speed the light beam of a laser in an image-forming apparatus such as a laser printer. By controlling the adhesion of dust on the mirror surfaces, a mirror which is thin in the axial direction, inexpensive to produce, and does not generate noise can be provided.